1. Field of the Invention
The present invention relates to techniques for estimating properties of cement in a borehole and, in particular, to estimating the amount of cement present at a particular location in the borehole.
2. Description of the Related Art
Downhole production boreholes utilized in subterranean operations typically utilize casings disposed therein to protect the borehole from downhole pressures, chemical reactions and other conditions and prevent failures such as borehole collapse, burst and tensile failures. Casings can also be used to define production zones in various portions of the borehole. In some cases, a layer of cement is deposited between the casing and a wall of the borehole. The cement may bond to either or both the casing and the borehole.
Casing monitoring and evaluation techniques are important tools in maintaining the integrity of the casing, and in turn maintaining the integrity of the borehole. Typical evaluation and maintenance procedures involve interrogating the casing and cement that is used to bond the casing to the borehole wall to determine whether voids are present between the casing and the borehole wall. The voids can be formed if the cement is damaged by chemicals used in the oil production process and result in incomplete or thin cement walls. Also, in some cases, the procedures utilized to deposit the cement can result in uneven distribution of the cement.
Typical methods for detecting cement voids include interrogating the casing and cement with acoustic signals to detect microannuli and other openings formed between the casing and the borehole wall. Detected microannuluses are typically corrected via methods such as applying hydrostatic pressure to the interior of the casing. Such detection methods can be inefficient and inaccurate in that they may be unable to effectively differentiate between an insignificant microannulus and a true void between the casing and the cement that can compromise the hydraulic seal formed therebetween.
Well logging is a technique used to perform measurements of an earth formation. In well logging, a logging instrument or tool is conveyed through a borehole penetrating the earth formation. The logging tool is configured to perform the measurements from within the borehole. Measurements are typically performed at various depths in the borehole. The measurements are associated with the depth at which they were performed to produce a log. Data from some of the logs may be used to determine relative amounts (i.e., saturation) of oil, gas or water in the pores of the earth formation.
At present, well logging is performed separately from the void detection process.